CONCEPT DEVELOPMENT OF A STEREOTACTIC HEAD FRAME FOR USE IN NEUROSURGERY

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CONCEPT DEVELOPMENT OF A STEREOTACTIC HEAD FRAME FOR USE IN NEUROSURGERY

CAMILLA ÖRNING

Master of Science Thesis Stockholm, Sweden 2011

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Concept Development of a Stereotactic Head Frame for Use in Neurosurgery

Camilla Örning

Master of Science Thesis MMK 2011:1 MKN 035 KTH Industrial Engineering and Management

Machine Design SE-100 44 STOCKHOLM

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Master of Science Thesis MMK 2011:1 MKN 035

Concept Development of a Stereotactic Head Frame for Use in Neurosurgery

Camilla Örning

Approved

2011-01-28

Examiner

Ulf Sellgren

Supervisor

Ulf Sellgren

Commissioner

Elekta Instruments AB

Contact person

Per Carlsson

Abstract

This report presents the development of a concept for a new stereotactic head frame for use in neurosurgery. The work has been performed at Elekta Instrument AB, which is one of the world's leading companies for the development of equipment that is used to treat cancer and other diseases of the brain. Elekta currently has a product called the Leksell Stereotactic System (LSS), which consists of a frame and arc, made in aluminium, that is used in stereotactic surgery. As diagnostic methods are changing and MRI uses more powerful magnetic fields, the need to use nonconductive or nonmagnetic material in the frame has arisen. The goal of this work was to develop a new fixation frame that would make improvements in the following areas: material, compatibility, design and patient comfort.

The work began with a background study and a study of users' anthropometric measurements. A QFD was conducted to develop customer requirements and engineering specifications. A function-means tree was created to study the product's functions. The means generated in the function-means-tree was then used to create concepts which were evaluated and a final solution was chosen.

A material seminar was held where the best materials were considered to be fiber glass reinforced epoxy manufactured in RTM with a lacquer finish. Some flaws were found with the chosen concept and as a result a new concept was created that in the end was selected for further development. The concept Doublebend’s, see figure 1, major advantages were its shape, which allowed entry to all parts of the brain, adjustable screw inserts and good opportunities for manufacturing. The frame’s deformation was analyzed and the results were used for the placement of the interface points to adjacent adapters. A MRI- adapter was developed and a prototype was produced in SLS.

The prototype was used for validation at a meeting with a neurosurgeon.

The conclusions of the work are that the developed concept has the potential to progress into a quality product, however much work remains. Further analysis, material testing and detailed design remain as future work.

Figure 1 Final concept Doublebend.

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Examensarbete MMK 2011:1 MKN 035

Konceptutveckling av en stereotaktisk huvudram för användning inom neurokirurgi

Camilla Örning

Godkänt

2011-01-28

Examinator

Ulf Sellgren

Handledare

Ulf Sellgren

Uppdragsgivare

Elekta Instruments AB

Kontaktperson

Per Carlsson

Sammanfattning

Detta examensarbete är ett produktutvecklingsprojekt som behandlar utvecklingen av ett koncept av en ny stereotaktisk huvudram för användning inom neurokirurgin. Arbetet har utförts i samarbete med Elekta Instrument AB som är ett av världens ledande företag för utveckling av utrustning som behandlar cancer och andra sjukdomar i hjärnan. Elekta har idag en produkt som heter Leksell Stereotactic System (LSS) och består av en ram och en båge, tillverkade i aluminium, som används vid stereotaktisk kirurgi. I och med att diagnosmetoderna förändras och att främst MRI använder kraftfullare magnetfält finns ett behov av att använda icke elektriskt ledande eller magnetiska material i ramen. Målet med arbetet var att ta fram en ny huvudfixeringsram som fram-för allt var förbättrad inom följande; material, kompatibilitet, design och patientkomfort.

Arbetet inleddes med en förstudie och en studie av användarnas antropometriska mått. En QFD genomfördes för att ta fram kundönskemål och krav på produkten. Ett funktions- medel-träd skapades för att studera produktens funktioner. De medel som skapats i funktions-medel-trädet användes sedan för att generera konceptlösningar som sedan utvärderades och en slutlig lösning valdes.

Ett materialseminarium hölls där bästa materialet ansågs vara glasfiber förstärkt epoxy tillverkad i RTM med en lackad yta.

En del brister fanns med det valda konceptet och som ett resultat skapades ett nytt koncept som i slutändan ansågs bättre och valdes för fortsatt utveckling. Konceptet Doublebend’s, se figur 1, fördelar var dess form, som tillät ingrepp till alla delar av hjärnan, vinklingsbara skruvinsatser och bra möjligheter till tillverkning. Ramens deformation analyserades och resultatet användes vid placering av låsningspunkter på ramen till angränsande adaptrar. En MRI- adapter togs fram och en prototyp tillverkades i SLS.

Prototypen användes för validering vid ett möte med en neurokirurg.

Slutsatserna från arbetet är att det framtagna konceptet har potential att utvecklas till en bra produkt, men att mycket arbete kvarstår. Djupare analyser, materialprov och

detaljkonstruktion återstår som framtida arbete. Figur 1 Slutliga konceptet Doublebend.

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PREFACE

This Master’s Thesis is the final project for receiving the Master of Science degree in Design and Product Development Engineering at the Royal Institute of Technology in Stockholm. It has been carried out at Elekta Instruments AB in Stockholm, Sweden, between August 2010 and January 2011.

I would like to take this opportunity to thank everyone at Elekta Instrument AB’s hardware group and product management team. Special thanks to my supervisor at Elekta, Marianne Plantz, for excellent guidance, feedback and encouragement throughout the project. Thanks also to Ulf Sellgren, supervisor at KTH ITM.

Camilla Örning Stockholm, January 2011

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NOMENCLATURE

This section explains the abbreviations used in the report as well as provide a glossary with medical terms for better understanding.

Abbreviations

 Magnetic Susceptibility CAD Computer Aided Design

CFRP Carbon Fibre Reinforced Plastic CT Computed Tomography

DBS Deep Brain Stimulations FEM Finite Element Method

GFRP Glass Fibre Reinforced Plastics LMPA Leksell Multi Purpose

Stereotactic Arc

LSS Leksell Stereotactic System M0 Magnetization

MRI Magnetic Resonance Imaging OR Operating Room

PET Positron Emission Tomography QFD Quality Function Deployment SRS Stereotactic Radio Surgery UD Unidirectional

X-ray Radiography

Glossary

Ablation The removal or destruction of tissue, part of the body or an abnormal growth.

AC-PC A line between two landmarks in the brain; Anterior Commissure (AC) and Posterior Commissure (PC), that are often used for navigation in stereotactic procedures.

Biopsy Removal and microscopic examination of tissue from a patient.

Joule heating The process by which the passage of an electric current through a conductor releases heat.

Lesion An area of tissue with an abnormal change in its texture or function, either as a result of injury or disease.

Posterior fossa The posterior cranial fossa is part of the intracranial cavity. It contains the brainstem and cerebellum, see figure 1.

Temporal lobe See green area marked in figure 1.

Figure 1 The structures of the brain. Source: Wikimedia Commons.

Temporal lobe

Frontal lobe

Parietal lobe

Occipitallobe

Cellebellum Brainstem

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1 INTRODUCTION

In this chapter the background, purpose, delimitations and methods used in this project are presented.

1.1 Background

Elekta Instruments AB is a company profiling in innovations and clinical solutions for treating cancer and brain disorders. The company was founded in 1972 by Lars Leksell, professor in Neurosurgery at the Karolinska Institute in Stockholm, Sweden. Elekta develops tools and treatment planning systems for open neurosurgery, radiation therapy and radiosurgery, as well as workflow enhancing software systems for cancer care (Elekta AB, 2010).

The field of neuroscience, i.e. the scientific study of the nervous systems, has progressed greatly during the second half of the 20^th century, to a great extent thanks to the advances in neurotechnology. Advents of brain imaging revolutionized the field and gave rise to a whole new shift in research. Scientists could now directly monitor the brain’s activities during experiments, which have lead to a significantly increased understanding of the human brain and its functions. Currently, modern science can image nearly all aspects of the brain as well as control a degree of the function of the brain (Wikipedia, Neurotechnology, 2010).

The knowledge of the nerve system has been vital in the development of neurosurgical treatment procedures for cancer and other common brain disorders. One technique of neurosurgery is stereotactic surgery; a minimally invasive form of surgical intervention, which makes use of a three-dimensional coordinates system to locate small targets inside the body. Once the target is located the surgeon could perform for example biopsy, injection, lesion, stimulation,

implantation, ablation (removal) and radiosurgery (SRS) (Wikipedia, Stereotactic Surgery, 2010).

One of Elekta’s products is the Leksell Stereotactic System (LSS), see figure 2, which is designed for both open and closed stereotactic neurosurgery. LSS is used in combination with imaging devices, such as MRI (Magnetic Resonance Imaging), CT (Computed Tomography) and x-ray (Radiography), to depict the brain in reference to the frame. Thus, each brain structure can be easily assigned a range of three coordinate numbers, which will be used for positioning the stereotactic device so the desired diagnostic or therapeutic procedure can be performed at the targeted area (Wikipedia,

Stereotactic Surgery, 2010). Figure 2 Leksell Stereotactic System. Source:

Elekta Instruments AB.

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The treatment procedures of stereotactic neurosurgery take place in a number of harsh environments, therefore the equipment in use need to withstand strong magnetic fields, gamma- and X-radiation and tough sterilization while being robust enough not to distort or shift during the day of treatment. In the design of a new stereotactic head frame all of the above mentioned factors must to be taken into account, while considering the patients’ and surgeons’ needs as well as the manufacturing process.

1.2 Purpose

The purpose of this Master Thesis project was to develop a design concept of a new stereotactic head frame. It was based upon previous work done by Elekta and will form the basis of their future development of an improved stereotactic system.

Professor Lars Leksell introduced the first stereotactic system in 1949. In 1989 the LSS was modernized, much due to MRI becoming a central part of stereotactic procedures, and to this date the frame has remained more or less the same. As treatment methods and technologies such as MRI become further advanced the need for a revised and modernized frame has arisen.

The new design concept had to make improvements in the following areas:

 Material: The frame must be inert in MRI, i.e. no conductive or magnetic material may be used.

 Compatibility: The frame’s design must be compatible with, for the specific treatment, required machines and surgery methods.

 Design: The frame must express Elekta’s design values.

 Patient comfort: The frame must not obstruct the patient during the day of treatment.

The design concept should also contribute to an overall improvement of the accuracy and safety of the whole system.

The aspiration of the work was that the design of the concept was thorough enough for a prototype (crafted in Rapid Prototyping) to be ordered and evaluated at the end of the project.

A report, presentation, CAD- and FEM-models as well as other useful information will be delivered to Elekta at the end of the project.

1.3 Delimitations

This project is a continuation of a project that was held at Elekta Instruments AB, and was put on hold. The work was based on the results of that project and was carried out in consultation with engineers at Elekta. Some of the work presented in this report originates from documents created by Elekta, and has been revised to serve the goals and method of this project.

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This project will mainly concern the frame in the Leksell Stereotactic System. The work does include some other parts of the system, which was considered necessary for the design of the frame.

The FEM simulations performed were simplified in respect to the material that was considered isotropic. Composite materials with fibres are orthotropic, which means that the material itself has to be modelled in able to perform a more thorough evaluation of the designs behaviour. This requires a great deal of knowledge and experience in FEM- simulations.

A suggestion for manufacturing method is presented, but no manufacturing specifications or drawings are made.

1.4 Method

The development process was comprised of eight elements; planning, literature study, requirement specification, concept generation, concept evaluation, 3D-modelling, analysis and validation.

The planning was done in a Gantt-chart and a risk analysis was performed where potential risks and preventions were determined. A literature studie was carried out to investigate how a stereotactic system is used to understand the needs from the patients, surgeons and Elekta as a company. Possible material choices and competitive solutions were also looked into. The information gathered formed the Frame of Reference.

A study of the previous work done at Elekta was also performed. This information forms the basis of the continued work.

A Quality Function Deployment (QFD) was made in order to understand the problem and generate engineering specifications. The model used was described by Ullman in The Mechanical Design Process (2003). The engineering specifications could then also be compiled into a list of product requirements. To be able to quantify some of the demands a study of human anthropometric measurements was performed. The goal of this study was to determent the variation of head sizes found in different groups of patients.

To further understand the problem a function analysis was carried out. The goal of the functional modelling was to decompose the problem into main functions and sub functions. The function analysis, formed in a shape of a tree, could then be used as a foundation for concept generation by adding means for providing the functions. This was a form of a morphological method (Ullman, 2003) to generate many concepts that each solves a single function, which could then be paired into a concept for the entire product.

Parallel with the concept generating, workshops with people who have knowledge about the product were held to aid the concept generation and evaluation. A workshop was also held in order to explore the possible choices of material as well as manufacturing methods.

The design concepts were evaluated based on a decision matrix, i.e. Pugh’s method (Ullman, 2003). The criterions were weighted using a method of comparing them with each other, described in Johannesson’s et al. Produktutveckling (2004). When a final

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design concept had been selected a product design phase began in order to refine the concept into a quality product. The chosen concept was designed in further detail in the CAD-software Solid Edge and evaluated in the FEM-software ANSYS. Parallel with the design process possible materials were evaluated and integrated into the design. A suggestion for manufacturing method was given.

A prototype crafted in Rapid Prototyping of the final concept was ordered and some validations were performed.

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2 FRAME OF REFERENCE

This chapter presents the theoretical reference frame and provides support and understanding for the remainder of the report.

2.1 Stereotactic Surgery

Stereotactic neurosurgery has always relied on technological advances to push the limit of the field. Unlike other subspecialties within neurosurgery, it does not focus on a disease but rather on a technology (Bakay, 1995). The breakthrough in stereotaxis came the late 1970’s, with the introduction of computed tomography (CT), long after the advent of the first stereotaxic apparatus in the beginning of the 20^th century (Gildenberg, 1998).

The fundamentals of stereotactic guidance are defining a coordinate system based upon the patient’s image, depicted in MRI, CT or PET, and the patient’s anatomy.

Subsequently a transformation and co registration of the two coordinate systems can be done to allow a point in space identified by the imaging technique to be related to the point of anatomy observed (Bakay, 1995).

Stereotactic surgery can be performed all over the patient’s body, but is mainly used for neurosurgical procedures in the head. This report will focus on surgery in the head, given that the frame in this project is to be designed for brain surgery.

The steps involved in the whole process of stereotactic surgery are presented in figure 3.

Figure 3 The steps involved in a typical stereotactic surgery.

2.1.1 Workflow – Stereotactic Surgery

Stereotactic surgery can be viewed as a process of moving through a 3-D coordinate system, beginning at point A (defined by a set of X, Y, Z coordinates), travelling to a point B, and performing a function C. That function can be transportation of material away from the site (biopsy, ventricular drainage), observation of the site (endoscopy, cortical mapping), destruction at the site (tumour ablation, functional lesioning), or transportation of material to the site (placing a stimulating electrode or drug polymer pellet) (Bucholz et al., 1998).

The majority of stereotactic surgeries utilize a frame to a) fixate the head to avoid image artifacts due to movement, and b) supply a coordinate system of the patient’s anatomy.

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The workflow of a typical stereotactic surgery where a frame is a part of the procedure is presented in the following section (Elekta Virtual Clinic, 2010).

1. Diagnosis and Decision of Treatment

Before every treatment the patient has to be diagnosed. This often involves a series of diagnostic tests followed by a consultation in which the outcome of the diagnostic investigation is discussed and a treatment plan is formed.

2. Fixation of Frame

The frame is attached to the patient’s head, see figure 4. LSS uses four self-tapping screws that are screwed into the fore- and back of head after local anaesthesia have been applied. The screws are tightened to make sure that the frame doesn’t alter its position during the day.

3. Imaging

The patient undergoes imaging; which can be done in MRI, x-ray, CT or PET. An indicator box with a built in reference system is mounted on the frame and produces reference points – fiducials – on images in all planes.

During scanning a frame adapter secures the patient’s head and ensures that it is supported. It also aligns the head horizontally which ease the process of comparing the images with stereotactic atlases. Figure 5 shows a patient being scanned in a MRI.

4. Target Determination

The doctor uses computer software to analyze the patient’s images and to plan and evaluate different surgical approaches, see figure 6.

The software calculates the target’s coordinates in reference to the frames coordinate system.

Figure 4 LSS frame is attached to the patient's head. Source: Elekta Instruments AB.

Figure 5 Magnetic resonance imaging with the coil placed around the head. Inside the coil the patient’s head is immobilized by the head frame.

References are provided by the indicator box mounted on the frame. Source: Elekta

Instruments AB.

Figure 6 Target determination using Elekta’s Surgery Plan. Source: Elekta Instruments AB.

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5. Setting of Coordinates

The Leksell Multi Purpose Stereotactic Arc (LMPA) is assembled and positioned on the frame, see figure 7. The coordinates that were calculated in the following step are set.

6. Treatment

Because the target has already been located, the treatment is minimally invasive and often there is only need for a burr hole. Depending on the diagnosis the surgery may be biopsies, injections and aspirations, evacuations, implantations, functional neurosurgery, pain treatment or stereotactic guidance. The patient could also be treated with radiosurgery or radiotherapy. A common stereotactic surgery where an electrode is implanted in the brain is shown in figure 8. It is common to use X-ray in the OR to check the position of e.g. electrodes and biopsy needles.

Figure 7 Assembly of LMPA and setting of coordinates. Source: Elekta Instruments AB.

Figure 8 Insertion of an electrode during deep brain stimulation. Source: Elekta Instruments AB.

2.1.2 Method of Treatment

Stereotactic surgery can be used to treat a diversity of disease, including movement disorders (e.g. Parkinson's disease and Tremor), pain, psychiatric conditions (e.g.

epilepsy and depression), tumors, cancer and lesion (Gildenberg, 1998).

Generally there are two categories of procedures for which stereotaxy is used (Quintiles Consulting, 2009):

- Stereotactic procedures that use a visible target on CT or MRI and include biopsy, evacuation (of cysts, abscesses, hematomas), removal and endoscopic stereotactic operations. Stereotactic radiosurgery and radiotherapy may also be included in this category;

- Functional stereotaxy that uses physiological targets that cannot be observed directly on CT or MRI and require physiological confirmation by various means.

Functional stereotaxy includes lesioning and deep brain stimulation (DBS) electrode implantations.

Entry Area

In stereotactic neurosurgery access to all parts of the brain is necessary. The surgeons enter the brain in different areas depending on what target they aim to reach. In figure 9 the entry areas are shown in green. The frame must allow access to these areas if all types of treatments are to be made possible.

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Figure 9 Entry areas to the brain (marked in green). Source: Elekta Instruments AB.

Screw Positions

The frame is attached to the skull using screws in the forehead and back of head. The best placements for the screws are displayed in green in figure 10. Areas that should be avoided are in the center of the forehead, in between the pupils where ventricles behind the skull bone increases the risk of the skull bone breaking from the force of the screws.

The area in the back of the head near the neck should also be avoided because of the muscular attachments located in that area. Screwing into those muscles causes extra pain to the patient.

Figure 10 Optimal placements of screws (marked in green). Source: Elekta Instruments AB.

There is sometimes also a need for alternative screw positions. If the patient has undergone brain surgery before the skull may be weakened in some areas, called bone flaps, which should be avoided in future surgery. The screws should be positioned perpendicular to the skull in order to avoid the screws from sliding when tightened.

The screw force varies depending on the patient and the user who attaches the frame.

2.1.3 Risks

As with any surgery, stereotactic procedures are associated with a number of risks.

Besides the general risk of surgery caused by clinical complications and anaesthesia there are some risks specific to the field of stereotactic treatments.

The main sources of instrument mediated risks in stereotactic surgery are (Quintiles Consulting, 2009):

 targeting errors;

- induced image distortion (associated with MRI imaging methods);

- image artifacts;

- deformation of the frame;

- application accuracy of the frame, arc and instrument carrier, etc;

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 intervention related risks, that limit accuracy, include;

- brain shift after preoperative planning,

- brain shift associated with the introduction of needles, electrodes etc., - variability of physiological function in relation to anatomical location,

etc.;

 other instrument related risks include;

- toxic reaction to materials of construction;

- toxic reaction due to unsterile instruments and/or operation environment;

- targeting errors due to mechanical deterioration of material.

2.2 The Leksell Stereotactic System

Professor Lars Leksell introduced the first stereotactic system in 1949. The system has been modified steadily over the years in order to be compatible with each advance in neurodiagnostic imaging (Lunsford et al., 1998). In 1989 the LSS was modernized, and today’s model G was created, much due to MRI becoming a central part of stereotactic procedures.

The success of the LSS has been great, with over 1800 system in use in 1300 clinics worldwide (Elekta Instruments AB, 2010), although, there are many improvements that can be made to advance the performance of the system. The following sections will describe the LSS and present some competitive systems.

2.2.1 System Structure

The fundamental idea of the LSS is the centre of arc principle. An arc attached to a frame is positioned so that the centre of the arc coincides with the target that has been identified in various imaging techniques, see figure 11. The only exception to the centre-of-arc concept within the Leksell stereotactic family of instruments is the Gamma Knife. Because the beam locations of the radiation sources are fixed, the patient is instead moved into the centre of focus by adjusting the stereotactic X, Y and Z positions of the target (Lunsford et al., 1998).

Figure 11 The centre-of-arc principle. Source: Elekta Instruments AB.

The LSS main parts are the frame and the arc, see figure 2 and figure 12, and is also complemented by adapters for mounting, and indicator boxes for reference points in scanning devices such as CT and MRI.

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Figure 12 The Leksell Stereotactic System’s frame and arc. Source: Elekta Instruments AB.

The Leksell Coordinate Frame

Application of the stereotactic frame requires that four self-tapping screws on adjustable fixations posts, be inserted under local aesthetic through the scalp to the outer table of the skull, see figure 13. After application, lifting up the frame ensures that the frame and head move together. In the vast majority of patients, the stereotactic head frame is applied within approximately 5 min (Lunsford et al., 1998).

The screws are tightened diagonally opposite each other, for better distribution of the induced stress in the frame. During fixation the frame is held at the correct height using earplugs that are temporarily placed in the external auditory canal. The front piece is exchangeable to provide flexibility – both with regard to access to the patient’s nose and mouth and in terms of frame positioning (Elekta Instruments AB, 2010).

Figure 13 Attachment of the frame to the patient's head. Source: Elekta Instruments AB.

The frame provides a Cartesian (X, Y, Z) coordinate system for the settings of the arc, see figure 14. The origin of the frame is located at the upper posterior right side of the frame and the centre of the frame is found at (100, 100, 100) (Lunsford et al., 1998).

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Figure 14 The LSS frame’s coordinate system. Source: Elekta Instruments AB.

The frame is shaped with grooves that permit rapid attachment of other components of the Leksell Stereotactic System, such as earplugs and the Leksell Multi Purpose Stereotactic Arc.

The LSS frame and arc are manufactured in anodized, which gives them the champagne colour.

The Leksell Multi Purpose Stereotactic Arc

The arc is designed to ensure that when a needle, probe, electrode or other micro- surgical instrument of standard Leksell length (190 mm) is correctly fitted, the active point of the instrument is always at the precise centre of the arc. Many different instruments can be fitted to the arc. The X, Y and Z coordinate settings are obtained from the patient’s treatment protocol after preoperative examination and target localization (Elekta Instruments AB, Instructions of use, 1998).

The semicircular arc permits accurate positioning of the instrument carrier at any point along its curve, which is engraved with the scale 0º to 170º. The arc may also be rotated around its point of fixation, thus providing even more flexibility of the angle of entry.

Indicator box

Before imaging an indicator box is mounted on the frame, see figure 15. The box provides fiducials that are detected by the scanner and therefore presented next to the depicted brain tissue on every image. Indicator boxes are provided for MRI, CT and X- ray.

Figure 15 MRI indicator box. Each indicator plate has fiducials consisting of tubes filled with Copper Sulphate solution. Source: Elekta Instruments AB.

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Adapter

Adapters are used to fixate the patient in the image modalities to ensure the patient doesn’t move during scanning and to align the indicator box correctly to the scanning plane. Movement often results in image artifacts, something that is unwanted. Elekta has a wide range of adapters to serve the many different kinds of MRI and CT devices available on the market.

2.2.2 Competing Systems

There are, besides Elekta, many other companies involved in the stereotactic treatment market, supplying both frame-based and frame-less systems. In this section some of the LSS competitors are listed and the qualities of their alternative solutions are presented, special consideration to the frame has been taken in the selection of presented products.

Zeppelin Micro Stereotactic System

The frame in this system is available in two options; in aluminium or fibre reinforced epoxy, with the later providing MRI compatibility, see figure 16. The posts are made in polyamide. The dimensions of the frame are too large for many MRI-coils (Zeppelin Medical Instruments Ltd., 2011).

Micromar Stereotactic AIMsystem

The frame can be mounted with or without posts, see figure 17. The product claims to be MRI and CT compatible, has exchangeable parts, interesting solutions for reach of entry areas (the arc supports can be turned upside down) and a nice, modern design. However, the frame’s dimensions are too big for the smallest MRI-coils on the market. The frame is made in aluminium and the posts in carbon fiber reinforced plastic (CFRP) or polyphenylene (Micromar, 2011).

IMRIS Surgery and Imaging Tools

IMRIS has a head fixation with a MRI-safe design that is fully integrated with the MRI-coil and OR table, see figure 18. Instead of fitting the frame inside the coil, they have designed a frame that fits around a specially designed coil.

The frame is made in CFRP, which they have been able to give a robust design since there is a lot of room to utilize outside the coil (IMRIS Inc., 2011).

Figure 16 The Zeppelin stereotactic frame.

Source: Zeppelin Medical Instruments Ltd.

Figure 17 The AIMsystem with and without posts. Source: Micromar.

Figure 18 The IMRIS head fixation and

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Renishaw

The company Renishaw has also built their own MRI-coil with an integrated fiducial system that is adapted to their frame, see figure 19. The frame is made in injection moulded plastic (Renishaw, 2011).

Inomed Stereotactic System

Inomed offers a ceramic ring shaped head frame for use in open surgery as well as stereotactic surgery, see figure 20. The open ring gives both patient and surgeon more freedom in the facial area. The ring is compatible with MRI and CT and claims to fit inside all smaller MRI-coils (Inomed Medizintechnik GmbH, 2011).

Figure 19 The Renishaw stereotactic head frame and MRI-coil. Source: Renishaw Ltd.

Figure 20 The Inomed stereotactic frame.

Source: Inomed Medizintechnik GmbH.

2.3 Imaging

3-D imaging is essential for a successful stereotactic procedure. There are many imaging technologies available to neurosurgeons today. These technologies vary in terms of the basis on which the brain is imaged, the resolution of the images produces, the accuracy of the imaging, and the effectiveness of the images for specific clinical application (Bucholz et al., 1998). Also the artifacts produced by stereotactic head fixating equipment vary depending on the imaging technique.

Some of the most common imaging techniques in stereotactic treatment are presented in further detail in the following sections.

2.3.1 Radiography (X-ray)

Radiography is a 2-D imaging technique that detects variations of density in the body.

Radiography is performed with an X-ray source on one side of the patient, and an X-ray detector on the other side. A short duration pulse of X-rays is emitted by the X-ray source and a large fraction of the X-rays interacts in the patient. Some of the X-rays pass through the patient and reach the detector, where a radiographic image is formed (Bushberg, 2002). Figure 21 shows an example of an X-ray image.

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Figure 21 Example of an x-ray image of the brain. Source: Wikimedia Commons.

Radiography is a 2-D imaging technique and cannot on its own be used to pinpoint a stereotactic target, thought it can be used intra-operative to check an approximate position of e.g. electrodes, needles or instruments. X-rays cannot penetrate material with high density. This has to be taken into account in the design of a new head frame. The frame must be designed in a) a low-density material that is not detected in the scan or b) in a way that it does not block any vital parts of the brain.

2.3.2 Computed Tomography (CT)

Computed tomography (CT) scanning, also called computerized axial tomography (CAT) scanning, is a medical imaging procedure that uses X-rays to show cross- sectional images of the body. While the patient is inside the opening of the CT imaging system, an X-ray source and detector within the housing rotate around the patient. The X-ray source produces a beam of X-rays that passes through a section of the patient's body. A detector opposite from the X-ray source records the X-rays that pass through the patient's body as an image. Many different images (at many angles through the patient) are collected during one complete rotation. For each rotation of the X-ray source and detector, the image data are sent to a computer to reconstruct all of the individual images into one or multiple cross-sectional images (slices) of the internal organs and tissues (FDA, 2010).

Figure 22 Example of a CT image. Source: Wikimedia Commons.

Since CT utilizes a scanning technique based upon almost the same physical phenomena as x-ray, a frame used in CT also has to be designed in a) a low-density material that is

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2.3.3 Magnetic Resonance Imaging (MRI)

The development of MRI has resulted in a significant improvement of anatomic resolution compared with CT. MRI is a 3-D imaging technique that is superior in visualizing individual components of the brain, which allows for precise stereotactic targeting and navigation. MRI uses instead of ionizing radiation, like CT, very strong magnetic fields (10,000 to 60,000 times stronger than the earth’s magnetic field) (Bushberg, 2002).

MRI utilizes the nuclear magnetic resonance properties of the proton – i.e., the nucleus of the hydrogen atom. When a person goes inside the powerful magnetic field of the scanner, the magnetic moment of some of the protons changes, and aligns with the direction of the field. A radio transmitter is briefly turned on; producing an electromagnetic field. This field has just the right energy, known as the resonance frequency, to change the spin directions of the protons. When the field is turned off the protons eventually returns to their original state of spin direction. The difference in energy between the two states releases an electromagnetic signal that the scanner detects. MRI uses the frequency of the returning signals to determine the position of each proton in the patient and can from that information construct an image. MRI produces a set of tomographic slices through the patient (Bushberg, 2002).

Because different types of tissue such as fat, white and grey matter in the brain, cerebral fluid and cancer all are comprised of different amounts of water, images made using MRI demonstrate high sensitivity to anatomic variations and therefore are high in contrast. Figure 23 shows an example of a MRI image of the brain (Bushberg, 2002).

Figure 23 Example of a MRI image. Source: Wikimedia Commons.

Image Artifacts

An image artifact is any feature that appears in an image that is not present in the original imaged object. It is important to be familiar with the appearance of artifacts, and try to reduce them, because artifacts can obscure, and be mistaken for, pathology.

Artifacts due to movement of object, material distorting the magnetic field, movement of body fluids during sequence are some of the reasons that can cause image artifacts (Hornak, 2010).

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When using the LSS, image artifacts for the most part appear due to the material in the frame, which is aluminium. Aluminium has a MRI magnetic compatibility of the first kind, which means that it can produce image distortion and degradation if it’s located close to the imaging region, such as in the case of stereotactic imaging (Schenck, 1995).

MRI-coil

During scanning a radio frequency receiver, also known as a coil, is placed around the head, see figure 5. Some coils can act as a radio frequency transmitter and /or receiver.

The closer the coil is to the imaged object, the better the outcome of the images will be.

In stereotactic imaging the frame most often is placed inside the coil. Currently the LSS and the indicator box cannot fit into some of the coils available, e.g. Siemens Magnetom and Phillips Achieva series, with a diameter of 240 and 250 mm (Elekta Instruments AB, MR Overview, 2010).

2.4 The Process of Cleaning

It is very important that equipment used in medical application is sterile, so as not to contaminate the patient, which can lead to a number of serious consequences. If the equipment is disposable it must be stored in sterile packaging until use. If the equipment is reusable it must be possible to clean with standard methods used in treatment facilities. The head frame is classified to have a high risk of infection due to its close contact to a break in the skin. Therefore simply cleaning and disinfection is not sufficient so the equipment needs to be sterilized as well (MHRA, 2010).

When the stereotactic head frame is mounted on the patient the frame is sterile from the sterilization process. During preoperative scanning the frame is in contact with a non- sterile environment and needs to be disinfected before surgery can begin. After surgery the frame is cleaned, sterilized and packed ready to be used again. An overview is presented in figure 3.

Common for all the steps included in the cleaning of medical equipment is that a good design can make a difference in the effectiveness of these methods. Small gaps and holes should be avoided and the shape should be easy to wipe off.

The most common methods of sterilizing the LSS are steam sterilization, EtO sterilization and STERRAD.

2.5 Material

2.5.1 Magnetic Susceptibility

Magnetic susceptibility () is a dimensionless quantitative measure of a material’s tendency to interact with and distort an applied magnetic field. The choice of material for instruments used in MRI-guided surgery is critical as the magnetic susceptibility determines, in part, the positional accuracy of MR images, and is also strongly related to the hazards associated with magnetic forces and torques (Schenck, 1995).

A materials magnetic susceptibility and magnetization are properties that determine the suitability of a material for use in or near an MR imaging system. Materials are

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traditionally classified into three categories with regard to their magnetic properties (Schenck, 1995);

- Hard magnetic materials have a nonzero, remaining magnetization (M00) and are banned from use in MRI.

- Soft magnetic materials are not magnetized unless they are subjected to an applied magnetic field. Their susceptibility is very large and they exhibit easily detected forces and torques in the presence of a strong magnetic field. Soft magnetic materials are not suitable for MRI.

- Nonmagnetic material has such small susceptibilities that no forces or torques are apparent inside a magnetic field. Not all nonmagnetic materials can be regarded as MRI compatible. It depends on how sensitive the MRI is and how small magnetic changes it can detect.

Materials can be classified into MRI compatible groups of the first and second kind, see table 1.

Table 1 MRI magnetic compatibility for MRI applications. water is taken as 9,05x10^-5 and is a close approximation to the susceptibility of human tissue. Source: Schenck, 1995.

Conditions Property Examples Comments

M₀0 and/or

>10^-2 MRI magnetic incompatibility

Iron, cobalt, magnetic stainless steel,

nickel

These materials experience strong magnetic forces and torques and create image distortion

and degradation even when they are located far from the imaging region.

M0<10⁴ A/m 10^-5<-water<10^-2

MRI magnetic compatibility of the first kind

Titanium, bismuth, nonmagnetic stainless steel

These materials do not experience easily detectable forces or torques, but they can produce obvious image distortion and degradation if they are located close to the

imaging region.

M0<10 A/m

-water<10^-5

MRI magnetic compatibility of

the second kind

Water, human tissue, copper,

zirconia

These materials produce no easily detected forces or torques and very limited or negligible

image distortion or degradation even when located close to the imaging region.

The distinction between MRI compatibility of the first and second kind is particularly relevant to the design of instruments for MRI-guided treatments, where the degree of image degradation that can be tolerated depends on the details of each surgical procedure (Schenck, 1995).

Some examples of different materials’ MRI compatibilities are presented in table 2 (Schenck, 1995). Worth noticing is that carbon fibre composites have very diverse compatibility depending on in which direction the fibre has in relation to the applied magnetic field. Fibreglass has proved to be of good MRI compatibility.

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Table 2 Materials magnetically compatible with MRI. Materials in the first group produce essentially no image abnormality. Materials in the second group produce noticeable image distortion, but for most applications it would not be significant. Materials in the third group produce obvious artifacts, but would

still be acceptable for many applications. Source: Schenck, 1995.

Group 1

<310^-6

Group 2

<10^-5

Group 3

<210^-4

Nylon Alumina (Al2O3) Bismuth

Silicon nitride (Si3N4) Silicon Aluminium

Teflon Air Graphite (polycrystalline)

Polysulfone Brass Carbon fibre composites ^b

Steatite Quartz (SiO2)

Carbon fibre composites ^a Lead

Vespel (acetal) Zinc

Zirconia (ZrO2) Plexiglass (PMMA) PEEK (poly-ether-ether-

ketone) Wood Copper

aApplied field parallel to graphite atomic planes.

bApplied field normal to graphite atomic planes.

A stereotactic head frame is positioned inside the imaging region very close to the image target. Therefore the ideal susceptibility is that of human tissue, rather than zero.

The goal is that an instrument introduced into the region of MR imaging should not disturb the pre-existing field (Schenck, 1995).

2.5.2 Electrical Conductivity

It is not only the materials magnetic susceptibility that determines a material’s MRI compatibility, the electrical conductivity also play a key role. The magnetic field in the MRI produces an electric field, which interacts with all electrically conducting materials within the imager (Schenck, 1995). This can cause a circulating flow of electrons, or a current, within the body of the conductor. These circulating eddies of current create induced magnetic fields that oppose the change of the original magnetic field. If the applied magnetic field, the electrical conductivity of the conductor or the speed of which the field that the conductor is exposed to increases – then the greater the currents developed and the opposing field becomes (Wikipedia, Eddy Curent, 2010). Induced currents in closed loop conductors are much larger then those in conductors without closed loops. In closed conducting circuits Joule heating can occur which can lead to burns on the patient where the screws are attached (Schenck, 1995).

Carbon fibre composite is electrically conductive which suggests that even though it may be magnetically compatible it may not be compatible for use in MRI. Glass fibre on the other hand is an excellent insulator.

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2.5.3 Effect of Irradiation

Irradiation can have a negative effect on materials, leading to for example reduced tensile strength. The stereotactic head frame is to be used in SRS, which is an environment with high gamma irradiation.

2.5.4 Chemical Resistance

It is very important that the material used in the head frame has a good chemical resistance since it must be able to cope with the different chemicals used in cleaning, disinfection and sterilization.

2.5.5 Composite Materials

Composites are materials that are comprised of a matrix that holds a fibrous reinforcing phase. The objective of the matrix is to bind the reinforcement together so efficiently that the reinforcement bears the introduced external load and to protect it from environmental effects. Polymer is the most common matrix and the most common reinforcements are glass, carbon and polymer. The composites reinforcement may be discontinuous (“short fibres”) and continuous “endless fibres”) and randomly oriented or aligned (Åström, 1997).

Mechanical Properties

While the matrix gives a composite its shape, surface appearance, environmental tolerance and overall durability; it is the fibrous reinforcement that carries most of the structural loads and thus largely dictates stiffness and strength (Åström, 1997).

Composites are often used in weight-critical applications thanks to their high specific strength (weight-to-strength ratio). Composite materials frequently have different properties in different directions, i.e. they are anisotropic. An unidirectional composite is significantly superior to metals as long as only the longitudinal directions is loaded.

When the transverse direction of a UD composite is loaded the relationship is the opposite and the composite basically has the same properties as those of its matrix. It is therefore common practise to orient the reinforcement in several directions to lessen this anisotropy (Åström, 1997).

Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. Creep can occur in polymers and metals which are considered viscoelastic materials. The stiffness of a polymer is time and temperature dependent. If a load is applied suddenly, the polymer responds like a hard solid. But if the load is then held constant, the molecules within the polymer may begin to rearrange and slide past one another, causing the polymer to gradually deform (Åström, 1997).

Manufacturing

Manufacturing is more critical for composites than for conventional construction materials, since material and component are normally manufactured simultaneously (Åström, 1997). Some of the most common manufacturing methods are described in the following section.

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 RTM – Resin Transfer Moulding

RTM is the most common liquid moulding technique that mainly owes its popularity to its capability of producing large, complex, and highly integrated components. The process uses a closed mould into which dry reinforcement is placed. It is possible to include inserts and fasteners, as well as cores to produce sandwich materials, within the reinforcement prior to impregnation. Liquid resin is injected into the mould. Crosslinking may then take place in ambient temperature or by accomplished by heating the mould (Åström, 1997).

 PrePreg

Preimpregnated fibres are used to make high-performance components.

Prepreg is delivered cooled and the crosslinking starts as the temperature rises. The prepregs are laid out against a mould and crosslinking take place either in room temperature or by heating the mould. Only one side of the component gets the same surface as the mould (Åström, 1997).

 Injection Moulding

Injection moulded parts often contain no reinforcement at all, but short-fibre reinforcements may be used to stiffen the component. Powdered or pelletized resin is put into the machine. A screw pushes the material forward and the resin is melted from the friction and heat created in the screw. The resin is pushed into the mould where the crosslinking takes place (Åström, 1997).

 Filament Winding

A prepreg tape is used and preheated on its way to a mandrel. At the contact point the previously wound layer is heated and the two molten surfaces merge. The shape of the component can be both convex and concave, however the latter requires some form of back tension. One great benefit with this technique is the elimination of secondary processing (crosslinking in oven) (Åström, 1997).

2.6 User Anthropometry

A product should be designed so that it is adapted to the user’s physical abilities and dimensions. During the development of a new product it is necessary to have access to data about the human body’s measurements, this is known as anthropometric data.

Anthropometry is the science of studying human dimensions, especially the size and shape of the body and its limbs. Applied anthropometry uses anthropometric data to design products and surroundings that humans use (Eklund et al., 1994).

The following questions are fundamental to applied anthropometry (Eklund et al., 1994):

 How shall we choose the “best” design solution to fit different users?

 When is it necessary to choose solutions with adjustable dimensions?

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To be able to answer these questions in a satisfying manner one must access the following information (Eklund et al., 1994):

 The users’ anthropometric properties.

 The restrictions these properties make on the design.

 The criteria that shall be applied to achieve an effective adaptation of the human and his/her surroundings.

Anthropometric data can be divided into percentiles. The X-th percentile is defined by the measurements of the subject who separates the X% part from the remainder.

Products are often designed in such manner that at least 90% of the population could use them without any restrictions or hindrances. As anthropometric tables indicate the measurements for female and males separately, the demand of 90% usability can be fulfilled by taking the values of the 5^thpercentile for the female and the 95^th percentile for the male (Karwowski 2006). If the product is to be used around the world, ethnicity also has to be taken into account, as different ethnic groups have differences in their bodily measurements (Eklund et al., 1994). Asians have for example more round heads then Caucasians, see figure 24, (Ball et al., 2010).

Figure 24Comparison of Chinese’s (red) and Caucasians’ (blue) head shapes in two anthropology databases. Source: A comparison between Chinese and Caucasian head shapes, 2010.

In the design of medical devices, the goal is that no one is to be denied treatment because of their physical constitution; therefore a better aim may be that 98% of the population can use the product. To achieve 98% usability the values of the 1^stpercentile for the female and the 99^th percentile for the male are to be used.

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2.7 Design - Elekta Design Values

For a product to be a premium product it must express certain values. Elekta has defined these values as key words; quality, patient friendly, trustworthy, accuracy, company identity and high-tech.

The key design values must be incorporated in the design process and should result in a head frame that (Elekta – Hamlet Design Book, 2009);

• gives a non-invasive expression.

• is made of composite materials.

• has no champagne colour to separate it from the LSS.

• has an organic shape with a light colour.

• does not cover the patient’s eye sight.

• has details that make the frame look trustworthy; fittings around pins, interface to adapter, product graphic, shape aligned with head anatomy.

• associate with head worn products – head phone, helmet.

• is a lightweight construction.

2.8 Standards and Regulations

There are some standards that the frame should conform to, e.g. standards for different sterilization methods such as EN554 - Steam sterilization, EN550 - EtO sterilization, ISO 11737-2 - STERRAD, ISO/DIS 11137-2- Radiation.

The frame must also conform to ISO10993-Biocompatibility and should be considered as MRI Safe according to FDA (Elekta Instruments AB, Pd125_120_01 Technical file).

There are also several other standards regarding information, packaging, safety etc. that don’t play a critical role in the design process and are therefore left out.

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3 THE DESIGN PROCESS

This chapter describes the conducted design process, from literature study to concept verification.

3.1 Literature Study

A literature study was carried out in the beginning of the project to collect information necessary to complete the task. Focus was placed on how the product was to be used, regarding the current stereotactic system, imaging, cleaning and material.

3.2 Ergonomic Study

An ergonomic study was carried out in order to determine the users’ anthropometric measurements and what sort of restrictions those may place on the design. The users in this case were the patients, since the patients are the ones who instigate the most limiting product dimensions. Anthropometric values for the 1^st and 99^th percentile, both men and women, were collected from ALBA, an anthropometric database. The range corresponds to 98 % of the population. The head width and length, see figure 25, were gathered for the available populations (Sweden, England, France, Poland, USA, Japan, India, Hong Kong). The product is to be used worldwide; therefore all ethnic populations should be presented. Since ALBA’s range of population is limited the study cannot be seen as complete but can still be used as guidance. The measured data is collected from adults.

Figure 25 Width and length of skull. Source: ALBA.

The results from the study are presented in table 3, and the data is presented in full detail in Appendix A . The largest head belongs to a male from Hong Kong and the smallest head to a female from India. The results correspond with the fact that Asians have more round shaped heads and Caucasians have more ellipse shaped heads (Ball et al., 2010).

Table 3 Maximum and minimum head width and length from the anthropometric study in ALBA Maximum Minimum

Head Width (mm) 175 125

Head Length (mm) 210 155

CONCEPT DEVELOPMENT OF A STEREOTACTIC HEAD FRAME FOR USE IN NEUROSURGERY

CONCEPT DEVELOPMENT OF A STEREOTACTIC HEAD FRAME FOR USE IN NEUROSURGERY

CAMILLA ÖRNING

Concept Development of a Stereotactic Head Frame for Use in Neurosurgery

Camilla Örning

Abstract

Sammanfattning

PREFACE

NOMENCLATURE

Abbreviations

Glossary

TABLE OF CONTENTS

1 INTRODUCTION

1.1 Background

1.2 Purpose

1.3 Delimitations

1.4 Method

2 FRAME OF REFERENCE

2.1 Stereotactic Surgery

2.2 The Leksell Stereotactic System

2.3 Imaging

2.4 The Process of Cleaning

2.5 Material

2.6 User Anthropometry

2.7 Design - Elekta Design Values

2.8 Standards and Regulations

3 THE DESIGN PROCESS

3.1 Literature Study

3.2 Ergonomic Study